1. Field of the Invention
The present invention relates generally to the production of mercaptans (alkanethiols), accompanied by the simultaneous recovery of hydrogen, from feedstreams containing hydrogen sulfide and hydrocarbons (alkanes) and, more particularly, it relates to a process for the production of methanethiol (methyl mercaptan) in a corona reactor from a feedstream containing hydrogen sulfide and methane in which hydrogen is continuously recovered from the gaseous mixture of products and reactants through a membrane wall.
2. Description of the Prior Art
The principal impetus behind the synthesis of thiols comes from the production of synthetic rubber. Thiols permit control of viscosity during polymerization of the synthetic rubber. Alkanethiols have also found extensive use in other areas, such as in the area of agricultural chemicals. In particular, methyl mercaptan (methanethiol) is commonly used for the manufacture of methionine, an amino acid used in chicken feed. Methyl mercaptan is also used to produce dimethyl sulfide; this chemical minimizes deposition in steam crackers, and is also used to desulfurize refinery products. Other uses include intermediate for jet fuel additives and fungicides.
In the past, three principal processes have been described for the production of methyl mercaptan:
Reaction of Methyl Alcohol (Methanol) with Hydrogen Sulfide
Reaction of methyl alcohol with hydrogen sulfide is the principal method used commercially. The principal reaction is as follows:CH3OH+H2S→CH3SH+H2OSeveral byproducts, including dimethyl sulfide, are also formed. The degree of methanol conversion increases with increase in temperature albeit at the cost of selectivity in favor of methyl mercaptan.
The catalysts in use today permit higher conversion and improved selectivity. The difference between various industrial processes lies primarily in the recipe of catalysts, operating temperature and pressure. The temperature generally lies in the range of two-hundred (200° C.) degrees Celsius to five-hundred (500° C.) degrees Celsius and the operating pressure ranges from one (atm) atmosphere to twenty-five (25 atm) atmosphere.
The principal disadvantage of this process, however, is that methanol is relatively expensive. In addition, product separation from reactants, and purification of methyl mercaptan requires fractionation. The presence of methanol changes the solubility of water in the organic phase. The fractionation to strip hydrogen sulfide from the effluent cannot be operated to obtain dry hydrogen sulfide as overhead, and dry methyl mercaptan as bottom product.
Reactions of Carbon Oxides with Hydrogen or Hydrogen Sulfide (in the Presence of Hydrogen)
The reactions of carbon oxides with hydrogen or hydrogen sulfide has been explored to reduce cost of raw materials such as methanol used in current commercial practice. The principal hydrogenation reactions, in the presence of sulfur, are as follows:CO+S+3H2→CH3SH+H2OCO2+S+4H2→CH3SH+2H2OAlternatively, in the presence of hydrogen sulfide, the principal reactions are as follows:CO+H2S→COS+H2 COS+3H2→CH3SH+H2OCO is the preferred reactant. Reactions involving CO2 are comparatively much slower; in addition, larger amounts of hydrogen are required. The difference between various processes lies primarily in the combination of catalysts, operating temperature and pressure. The temperature generally lies in the range of two hundred and fifty (250° C.) degrees Celsius and four hundred (400° C.) degrees Celsius and the operating pressure, in excess of ten (10 atm) atmospheres, ranges preferably from thirty (30 atm) atmospheres to seventy (70 atm) atmospheres.
The principal disadvantage of this process is the requirement for hydrogen and/or carbon monoxide. Steam reforming of methane may be necessary to provide the raw materials. As noted earlier, use of carbon dioxide increases the hydrogen requirement. The principal byproducts include COS, CS2, CH4. Product separation from reactants remains an issue especially with reference to methane. Finally, the operating pressure is significantly higher than that required for synthesis of methyl mercaptan from methanol.
Hydrogenation of Carbonyl Sulfide or Carbon Disulfide
Methyl mercaptan can be produced through the hydrogenation of carbonyl sulfide according to the following:CS2+3H2→CH3SH+H2SThis catalytic reaction is carried out at temperatures between one hundred and fifty (150° C.) and three hundred and fifty (350° C.) degrees Celsius; the operating pressure is between ten (10 atm) atmospheres and fifty (50 atm) atmospheres. The major disadvantage of this process is the requirement for hydrogen as well as carbon disulfide. Production of hydrogen sulfide poses an additional problem even though its presence appears to assist the reaction. A variation on this approach, as follows:CS2+CO2→2COSCOS+3H2→CH3SH+H2Oand avoids production of hydrogen sulfide. However, inexpensive sources of carbon disulfide and hydrogen remain elusive, but necessary.